In the Help section it says a bounty period is 7 days and points will be awarded based on most up voted answers. It is a slow time of the year for gardening in the Northern hemisphere and my question here has not received any answers that provide references.

If the bounty period could be extended to 14 days then others might participate.

1 Answer
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Unfortunately, there's no way to extend a bounty past the initial seven days. Placing a bounty on a question is analagous to buying advertising time on TV: you don't get a refund or extra air time if everybody is watching the Superbowl on another channel when your advertisement is aired.

After this bounty expires, you can always place another bounty on the question, though the cost will double every time you do that up to the maximum of 500 points (source).